Radiation therapy, also known as radiotherapy, is used to treat tumors and other ailments in mammalian (e.g., human and animal) tissue. In a radiotherapy treatment session, a high-energy beam is applied from an external source towards a patient to produce a collimated beam of radiation directed to a target site of a patient. The target may be a region of the patient's body that contains a diseased organ or tumor that is to be exposed to, and treated by, the radiation beam. The placement and dose of the radiation beam must be accurately controlled to ensure that the target receives the dose of radiation that has been prescribed for the patient by a physician. The placement of the beam should be such that it minimizes damage to the surrounding healthy tissue, often called the organ(s) at risk (OARs).
One way to improve the accuracy of the beam placement is by the acquisition of one or more medical images of the patient in the intended treatment position. Such images are known as planning images. The planning images are acquired prior to a radiotherapy treatment session, and are often acquired many days before the treatment session.
Physicians can use the planning images to identify and contour a target as well as OARs. Contouring can be performed manually, semi-automatically, or automatically. A treatment contour, often referred to as a planned target volume (PTV), is created which includes the target contour plus sufficient margins to account for microscopic disease as well as treatment uncertainties. A radiation dose is prescribed by the physician, and a radiotherapy treatment plan is created that optimally delivers the prescribed dose to the PTV while minimizing dose to the OARs and other normal tissues. The treatment plan can be generated manually by the physician, or can be generated automatically using an optimization technique. The optimization technique may be based on clinical and dosimetric objectives and constraints (e.g., the maximum, minimum, and mean doses of radiation to the tumor and OARs).
A treatment course is developed to deliver the prescribed dose over a number of fractions, wherein each fraction is delivered in a different treatment session. For example, 30-40 fractions are typical, but five or even one fraction can be used. Fractions are often delivered once, or in some cases twice, per weekday. In some cases, the radiation treatment plan can change throughout the course to focus more dose in some areas.
In each fraction, the patient is set up on a patient support accessory (often a “couch”) of a radiotherapy device, and repositioned as closely as possible to their position in the planning images. This is a difficult task to carry out accurately in practice, because the patient is not a rigid object and the patient's anatomy can move. Fraction-to-fraction motions are often referred to as interfractional motion, while motion occurring during a fraction itself is often referred to as intrafractional motion.
Image-guided radiotherapy (IGRT) attempts to solve the problem of interfractional motion. IGRT involves acquiring one or more medical images of the patient shortly before radiotherapy (often referred to as “daily images”), and using those images to identify and compensate for interfractional motion. As opposed to planning images, which can be acquired on any diagnostic scanner, IGRT images are acquired directly in the treatment room, while the patient is in the treatment position. To compensate for interfractional motion, IGRT images are compared with the planning images to quantify changes in the patient's anatomy that have occurred since the planning images were generated. For example, the planning images and IGRT images may be analyzed to calculate a global shift and/or rotation that maps the planning images to the IGRT images. Once the shift and/or rotation have been calculated, a corresponding adjustment to the position of the patient support accessory can be made, such that the position of the patient during the treatment session more closely matches the position of the patient when the planning images were acquired.
Adaptive radiotherapy is another technique that aims to solve the problem of interfractional motion. As with IGRT, adaptive radiotherapy involves acquiring one or more medical images of the patient shortly before a radiotherapy treatment session, and using those images to identify and compensate for interfractional motion. In adaptive radiotherapy, the planning images and the images taken shortly before the treatment session may be analyzed to generate a deformation vector field (DVF). The DVF is a matrix whose elements are vectors, and in which each vector defines a geometric transformation to map a voxel in a planning image to a corresponding voxel in an image taken shortly before the treatment session. The DVF can be used to transform the spatial distribution of the radiation dose prescribed by a treatment plan, in order to compensate for changes in the patient's anatomy that have occurred since the planning images were acquired.
Transforming the dose distribution in this manner may result in the target receiving less than the prescribed dose and/or an OAR being exposed to a higher level of radiation than the physician intended. There is thus a need to verify that a transformed dose distribution is clinically effective and safe.